Class ReentrantLock

A reentrant mutual exclusion Lock with the same basic
behavior and semantics as the implicit monitor lock accessed using
synchronized methods and statements, but with extended
capabilities.

A ReentrantLock is owned by the thread last
successfully locking, but not yet unlocking it. A thread invoking
lock will return, successfully acquiring the lock, when
the lock is not owned by another thread. The method will return
immediately if the current thread already owns the lock. This can
be checked using methods isHeldByCurrentThread(), and getHoldCount().

The constructor for this class accepts an optional
fairness parameter. When set true, under
contention, locks favor granting access to the longest-waiting
thread. Otherwise this lock does not guarantee any particular
access order. Programs using fair locks accessed by many threads
may display lower overall throughput (i.e., are slower; often much
slower) than those using the default setting, but have smaller
variances in times to obtain locks and guarantee lack of
starvation. Note however, that fairness of locks does not guarantee
fairness of thread scheduling. Thus, one of many threads using a
fair lock may obtain it multiple times in succession while other
active threads are not progressing and not currently holding the
lock.
Also note that the untimed tryLock method does not
honor the fairness setting. It will succeed if the lock
is available even if other threads are waiting.

It is recommended practice to always immediately
follow a call to lock with a try block, most
typically in a before/after construction such as:

In addition to implementing the Lock interface, this
class defines methods isLocked and
getLockQueueLength, as well as some associated
protected access methods that may be useful for
instrumentation and monitoring.

Serialization of this class behaves in the same way as built-in
locks: a deserialized lock is in the unlocked state, regardless of
its state when serialized.

This lock supports a maximum of 2147483647 recursive locks by
the same thread. Attempts to exceed this limit result in
Error throws from locking methods.

Constructor Detail

ReentrantLock

Creates an instance of ReentrantLock.
This is equivalent to using ReentrantLock(false).

ReentrantLock

public ReentrantLock(boolean fair)

Creates an instance of ReentrantLock with the
given fairness policy.

Parameters:

fair - true if this lock should use a fair ordering policy

Method Detail

lock

public void lock()

Acquires the lock.

Acquires the lock if it is not held by another thread and returns
immediately, setting the lock hold count to one.

If the current thread already holds the lock then the hold
count is incremented by one and the method returns immediately.

If the lock is held by another thread then the
current thread becomes disabled for thread scheduling
purposes and lies dormant until the lock has been acquired,
at which time the lock hold count is set to one.

tryLock

public boolean tryLock()

Acquires the lock only if it is not held by another thread at the time
of invocation.

Acquires the lock if it is not held by another thread and
returns immediately with the value true, setting the
lock hold count to one. Even when this lock has been set to use a
fair ordering policy, a call to tryLock()will
immediately acquire the lock if it is available, whether or not
other threads are currently waiting for the lock.
This "barging" behavior can be useful in certain
circumstances, even though it breaks fairness. If you want to honor
the fairness setting for this lock, then use
tryLock(0, TimeUnit.SECONDS)
which is almost equivalent (it also detects interruption).

If the current thread already holds this lock then the hold
count is incremented by one and the method returns true.

If the lock is held by another thread then this method will return
immediately with the value false.

tryLock

Acquires the lock if it is not held by another thread within the given
waiting time and the current thread has not been
interrupted.

Acquires the lock if it is not held by another thread and returns
immediately with the value true, setting the lock hold count
to one. If this lock has been set to use a fair ordering policy then
an available lock will not be acquired if any other threads
are waiting for the lock. This is in contrast to the tryLock()
method. If you want a timed tryLock that does permit barging on
a fair lock then combine the timed and un-timed forms together:

if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }

If the current thread
already holds this lock then the hold count is incremented by one and
the method returns true.

If the lock is held by another thread then the
current thread becomes disabled for thread scheduling
purposes and lies dormant until one of three things happens:

If the specified waiting time elapses then the value false
is returned. If the time is less than or equal to zero, the method
will not wait at all.

In this implementation, as this method is an explicit
interruption point, preference is given to responding to the
interrupt over normal or reentrant acquisition of the lock, and
over reporting the elapse of the waiting time.

unlock

public void unlock()

Attempts to release this lock.

If the current thread is the holder of this lock then the hold
count is decremented. If the hold count is now zero then the lock
is released. If the current thread is not the holder of this
lock then IllegalMonitorStateException is thrown.

When the condition waiting
methods are called the lock is released and, before they
return, the lock is reacquired and the lock hold count restored
to what it was when the method was called.

If a thread is interrupted
while waiting then the wait will terminate, an InterruptedException will be thrown, and the thread's
interrupted status will be cleared.

Waiting threads are signalled in FIFO order.

The ordering of lock reacquisition for threads returning
from waiting methods is the same as for threads initially
acquiring the lock, which is in the default case not specified,
but for fair locks favors those threads that have been
waiting the longest.

getHoldCount

public int getHoldCount()

Queries the number of holds on this lock by the current thread.

A thread has a hold on a lock for each lock action that is not
matched by an unlock action.

The hold count information is typically only used for testing and
debugging purposes. For example, if a certain section of code should
not be entered with the lock already held then we can assert that
fact:

the number of holds on this lock by the current thread,
or zero if this lock is not held by the current thread

isHeldByCurrentThread

public boolean isHeldByCurrentThread()

Queries if this lock is held by the current thread.

Analogous to the Thread.holdsLock(java.lang.Object) method for built-in
monitor locks, this method is typically used for debugging and
testing. For example, a method that should only be called while
a lock is held can assert that this is the case:

isFair

getOwner

Returns the thread that currently owns this lock, or
null if not owned. When this method is called by a
thread that is not the owner, the return value reflects a
best-effort approximation of current lock status. For example,
the owner may be momentarily null even if there are
threads trying to acquire the lock but have not yet done so.
This method is designed to facilitate construction of
subclasses that provide more extensive lock monitoring
facilities.

Returns:

the owner, or null if not owned

hasQueuedThreads

public final boolean hasQueuedThreads()

Queries whether any threads are waiting to acquire this lock. Note that
because cancellations may occur at any time, a true
return does not guarantee that any other thread will ever
acquire this lock. This method is designed primarily for use in
monitoring of the system state.

Returns:

true if there may be other threads waiting to
acquire the lock

hasQueuedThread

Queries whether the given thread is waiting to acquire this
lock. Note that because cancellations may occur at any time, a
true return does not guarantee that this thread
will ever acquire this lock. This method is designed primarily for use
in monitoring of the system state.

getQueueLength

public final int getQueueLength()

Returns an estimate of the number of threads waiting to
acquire this lock. The value is only an estimate because the number of
threads may change dynamically while this method traverses
internal data structures. This method is designed for use in
monitoring of the system state, not for synchronization
control.

Returns:

the estimated number of threads waiting for this lock

getQueuedThreads

Returns a collection containing threads that may be waiting to
acquire this lock. Because the actual set of threads may change
dynamically while constructing this result, the returned
collection is only a best-effort estimate. The elements of the
returned collection are in no particular order. This method is
designed to facilitate construction of subclasses that provide
more extensive monitoring facilities.

Returns:

the collection of threads

hasWaiters

Queries whether any threads are waiting on the given condition
associated with this lock. Note that because timeouts and
interrupts may occur at any time, a true return does
not guarantee that a future signal will awaken any
threads. This method is designed primarily for use in
monitoring of the system state.

getWaitQueueLength

Returns an estimate of the number of threads waiting on the
given condition associated with this lock. Note that because
timeouts and interrupts may occur at any time, the estimate
serves only as an upper bound on the actual number of waiters.
This method is designed for use in monitoring of the system
state, not for synchronization control.

getWaitingThreads

Returns a collection containing those threads that may be
waiting on the given condition associated with this lock.
Because the actual set of threads may change dynamically while
constructing this result, the returned collection is only a
best-effort estimate. The elements of the returned collection
are in no particular order. This method is designed to
facilitate construction of subclasses that provide more
extensive condition monitoring facilities.